I was hoping anyone could give me some advice on the dumble style PI trimmer that feeds B+ voltage to the PI plates.. I have installed the trimmer on my amp after doing upgrades and have been thrilled with the results and finally am trying this one.. I hooked it up copying the Dumble 102 design. So I have 110K and 120K plates.. and have my B+ Voltage of around 425V feeding the wiper of the 10K trimmer pot, then each side of the trimmer pot feeds each plate resistor.
Theres just something I am confused about and I read people saying get the plates around 6-7 VDC apart then tweak by ear. Only I've found that the closest I can get my plates is 272V on one side (120K plate) and 290V approx on the other plate (110K). Am I missing something here and is there more to consider?? I dont understand how I cant get them close considering Ive copied the schematic exactly...
ANy input most appreciated!
Cheers
diddymix wrote: ↑Mon Sep 16, 2019 7:49 pm
Hi guys,
I was hoping anyone could give me some advice on the dumble style PI trimmer that feeds B+ voltage to the PI plates.. I have installed the trimmer on my amp after doing upgrades and have been thrilled with the results and finally am trying this one.. I hooked it up copying the Dumble 102 design. So I have 110K and 120K plates.. and have my B+ Voltage of around 425V feeding the wiper of the 10K trimmer pot, then each side of the trimmer pot feeds each plate resistor.
Theres just something I am confused about and I read people saying get the plates around 6-7 VDC apart then tweak by ear. Only I've found that the closest I can get my plates is 272V on one side (120K plate) and 290V approx on the other plate (110K). Am I missing something here and is there more to consider?? I dont understand how I cant get them close considering Ive copied the schematic exactly...
ANy input most appreciated!
Cheers
That should be 6-7VAC not DC. I.e. you're adjusting how 'balanced' the signal output swings. If you have it perfectly balanced, it's more sterile, clean and the like, which is great for HIFI, if you instead imbalance it a touch, you get more second order harmonic distortion, which is what makes a guitar amp sing more.
I.e. you may have about 55 to 60 VAC coming out of the PI into the power tubes, or what not, and you want it intentionally imbalanced a bit. To set this you need to send in a known signal, say a 1khz 100mV signal at the input and then do the tweaking from there. Either a DMM set to AC volts or an oscilloscope set to look at the different waveforms, with the second 'inverted' so they line up and you can try to get the imbalance set to that 6-7VAC sweet spot.
diddymix wrote: ↑Mon Sep 16, 2019 7:49 pm
Hi guys,
I was hoping anyone could give me some advice on the dumble style PI trimmer that feeds B+ voltage to the PI plates.. I have installed the trimmer on my amp after doing upgrades and have been thrilled with the results and finally am trying this one.. I hooked it up copying the Dumble 102 design. So I have 110K and 120K plates.. and have my B+ Voltage of around 425V feeding the wiper of the 10K trimmer pot, then each side of the trimmer pot feeds each plate resistor.
Theres just something I am confused about and I read people saying get the plates around 6-7 VDC apart then tweak by ear. Only I've found that the closest I can get my plates is 272V on one side (120K plate) and 290V approx on the other plate (110K). Am I missing something here and is there more to consider?? I dont understand how I cant get them close considering Ive copied the schematic exactly...
ANy input most appreciated!
Cheers
That should be 6-7VAC not DC. I.e. you're adjusting how 'balanced' the signal output swings. If you have it perfectly balanced, it's more sterile, clean and the like, which is great for HIFI, if you instead imbalance it a touch, you get more second order harmonic distortion, which is what makes a guitar amp sing more.
I.e. you may have about 55 to 60 VAC coming out of the PI into the power tubes, or what not, and you want it intentionally imbalanced a bit. To set this you need to send in a known signal, say a 1khz 100mV signal at the input and then do the tweaking from there. Either a DMM set to AC volts or an oscilloscope set to look at the different waveforms, with the second 'inverted' so they line up and you can try to get the imbalance set to that 6-7VAC sweet spot.
~Phil
Ah OK thanks Phil! I had no idea people were talking about the audio signal VAC.. DO you know if theres a rough guidline on how much Voltage DC difference I want on the plates?? Perhaps if theres a close ish figure to get me going I could use that then tweak by ear... I dont have any means of injecting an accurate 1KHz tone and measuring.. I just have a multimeter and my guitar!
You don't need to worry about the VDC. You can put the multimeter in AC mode and measure the voltage. The VDC won't give you any data on the balance of the triodes anyway, as they could be imbalanced and conducting different amounts at random different voltages.
As for a means, do you have a smart phone? there are tons of signal gen apps and then you'd just need a mini jack to 1/4" adapter to put it into the amp. You can use the DMM to measure the AC input to be between 100 and 400mV so you're not pegging the PI.
diddymix wrote: ↑Mon Sep 16, 2019 7:49 pm
Hi guys,
I was hoping anyone could give me some advice on the dumble style PI trimmer that feeds B+ voltage to the PI plates.. I have installed the trimmer on my amp after doing upgrades and have been thrilled with the results and finally am trying this one.. I hooked it up copying the Dumble 102 design. So I have 110K and 120K plates.. and have my B+ Voltage of around 425V feeding the wiper of the 10K trimmer pot, then each side of the trimmer pot feeds each plate resistor.
Theres just something I am confused about and I read people saying get the plates around 6-7 VDC apart then tweak by ear. Only I've found that the closest I can get my plates is 272V on one side (120K plate) and 290V approx on the other plate (110K). Am I missing something here and is there more to consider?? I dont understand how I cant get them close considering Ive copied the schematic exactly...
ANy input most appreciated!
Cheers
That should be 6-7VAC not DC. I.e. you're adjusting how 'balanced' the signal output swings. If you have it perfectly balanced, it's more sterile, clean and the like, which is great for HIFI, if you instead imbalance it a touch, you get more second order harmonic distortion, which is what makes a guitar amp sing more.
I.e. you may have about 55 to 60 VAC coming out of the PI into the power tubes, or what not, and you want it intentionally imbalanced a bit. To set this you need to send in a known signal, say a 1khz 100mV signal at the input and then do the tweaking from there. Either a DMM set to AC volts or an oscilloscope set to look at the different waveforms, with the second 'inverted' so they line up and you can try to get the imbalance set to that 6-7VAC sweet spot.
No, it is a DC voltage difference at the PI plates you are looking for initially, with the input triode being higher by ~5V. This will be the case if the triodes are matched, and the plate loads are ~10k apart. If you can't get the DC voltages into that range, you could use a larger trimmer, or get a tube with matched triodes. The AC outputs will also be approximately equal at this point. It is also recommended that the signal be applied to the PI input when you use the trimmer to balance the AC voltage at the PI outputs, or the power tube cathodes, which will cancel out any AC miss-match. Check the thread on adjusting the PI trimmer in the Dumble discussion section. You do need a signal source, but you can use a multimeter to balance the outputs.
Oh odd, I could have sworn someone else in this forum told me to set it around 6-7VAC adn i even stated that in another thread and nobody countered it as 'wrong' sorry. The 'right' way per what I'm reading trying to find where someone 'purportedly' told me this led to Gil's explanation here:
jelle wrote: ↑Tue Sep 17, 2019 3:22 pm
Match for close AC signal output at the plates.
That would be to get it balanced equally, I think the discussion topic is taht a slight imbalance is good for 2nd order harmonics. Or has it been your experience that dumble was shooting for perfectly balanced PI?
My preference is to use such a balance trimmer to maximise the clean power output; scope the speaker output and at the onset of clipping, tweak the balance to even up the degree of clipping for the upper and lower waves.
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Was already mentioned I believe although every one described a personal preference
SVT or the Ampeg method which boils to feeding an input signal of 40 Hz and dialing the output to X volts AC RMS level depending on the designated power amp rating (at min load) and then balancing the PI for 0 DC difference (or minimal harmonic distortion at the output), which both aim at what you can call max clean power, I guess. Harmonic distortion method is kinda more accurate, but requires specialized equipment.
I found that the reason my voltages wouldnt get close was because my tube didnt have matched triodes. Now Ive got a new JJ in there and it works a charm,.. I have to do it by ear though because I dont have distortion analyzers etc.. but I can hear and feel the effect as I tweak the trimmer, you DO get the bloom and swell when you find it, and it really improves the feel and touch of the amp. much more jump off the notes.. and feels more horn like as you play legato lines, thank you!
